fix: remove tests on single variable getter

Author: FabianHofmann

test/test_variable.py

@@ -74,7 +74,6 @@ def test_wrong_variable_init(m: Model, x: linopy.Variable) -> None:
 
 def test_variable_getter(x: linopy.Variable, z: linopy.Variable) -> None:
     assert isinstance(x[0], linopy.variables.Variable)
-    assert isinstance(z[0], linopy.variables.Variable)
 
     assert isinstance(x.at[0], linopy.variables.Variable)